In the manufacturing of corrugated paperboard, a single facer apparatus is used to corrugate or flute the medium web, to apply glue to the flute tips on one face of the fluted medium web, to bring a liner web into contact with the glued flute tips on the medium web, and apply heat downstream with the liner web in contact with the glued flute tips on the medium web to provide an initial bond. The medium web is fed between inter-engaging corrugating rolls to corrugate the medium web. It is important, especially at high machine speeds, that the temperature and moisture content of the medium web prior to corrugation be sufficient to enhance the pliability of the web during the corrugating process. If the medium web is too brittle, the inter-engaging corrugating rolls are more likely to rip or burst the medium web, especially as machine speeds are increased. It is generally realized in the art that the optimum condition for the medium web prior to corrugation is approximately 212.degree. F., and 10% moisture by weight, however these conditions can vary with the application. It is therefor relatively common in the art to preheat the medium web and/or moisten the web with steam showers before corrugating the medium web.
In the art, it is common to feed the medium web around a pre-heated drum before corrugating. The tension of the medium web holds the drum. The wrap around the drum is adjusted to promote heat transfer as desired. In a prior system developed by the assignee of this application, the pre-heated drum is replaced by a hot plate assembly in which a vacuum is used to draw the medium web against the heated surface of the hot plates. More specifically, the hot plate assembly includes a plurality of hot plate tubes defining a generally planar heat transfer contact area against which the medium web is drawn against by the vacuum. Steam is passed through each of the tubular hot plates to heat the hot plates, preferably to approximately 375.degree. F. Using the vacuum hot plate assembly is much simpler than using a pre-heated rotating drum. While the vacuum hot plate assembly has been generally effective for pre-heating the medium web, in some applications the vacuum hot plate assembly removes more moisture from the medium web than is desirable before corrugating. This is also true with other preheaters. It is therefore desirable to modify the hot plate assembly so that the vacuum does not remove too much moisture. Also, at high production speeds, it is sometimes difficult to achieve sufficient heat transfer to the medium web to optimally pre-condition the web.